Do as Uncle Al suggests and check the stator and CDI unit. I assume you have a manual start tiller control engine. You should have a stop switch in the tiller and the emergency lanyard stop switch in the lower cowl. Run the engine as normal and disconnect the two Black/Yellow bullet connectors to the CDI unit from the stop switches. See if the engine runs OK now. Have had problems in the past with shorting of one of the stop switches with older engines. To stop the engine after this test disconnect the fuel and let it run out, pull the primer/choke on or connect the wires back up and stop on the switch.